Copying and Pasting Fcurve modifiers Through Python
I don't think it is possible to get/set the active fcurve / driver. So you can not override the context for bpy.ops.graph.fmodifier_copy

You might use from_existing and adjust the data_path attribute from the returned driver accordingly if you want to copy the driver or try out this script:

import bpy

sk = bpy.data.shape_keys['Key']

source = sk.animation_data.drivers[0]
target = sk.animation_data.drivers[1]

for mod in source.modifiers:
    copy = target.modifiers.new(mod.type)
    for prop in mod.bl_rna.properties:
        if not prop.is_readonly:
            setattr(copy, prop.identifier, getattr(mod, prop.identifier))